Electrical production of musical tones by electrostatic means



y 1940- A. H. MIDGLEY 2,207,489

ELECTRICAL PRODUCTION OF MUSICAL TONES BY ELECTROSTATIC MEANS Filed Sept. 21, 1957 2 Sheets-Sheet 1 CCC July 9, 1940. A MlDGLEY 2,207,489

ELECTRICAL PRODUCTION OF MUSICAL TONES BY ELECTROSTATIC MEANS Filed Sept. 21, 1957 2 Sheets-Sheet 2 16:: an saw 4n 42 m 2n Fm Patented July 9, 1940 PATENT. OFFICE ELECTRICAL PRODUCTION F MUSICAIL TONES BY ELECTROSTATIC MEANS I Albert Henry Midgley, Wembley, England Application September 21. 1937, Serial No. 164,948 In Great Britain September 23, 1938 9 Claims.

This invention relates to sound reproducing apparatus of the kind in which an undulating potential is produced electrostatically as by the relative rotation of charged and uncharged elec= a trodes either or both of which is or are of undulating form and in which the charging of the charged electrodes is controlled by the operation of keys controlling key contacts, the arrangement being such that when any key is depressed se- 10 lected electrodes to be charged for the partials of the note corresponding to the key actuated, are connected to a source of potential, the selection being according to the stop or stops drawn.

When two or more keys are simultaneously wl6 pressed or two or more stops are drawn or when two or more keyboards are played simultaneously any one electrode may be connected to 'a number of potential sources. The resultant potential on the electrode will therefore be the mean of 20 these potentials and therefore the sound produced will not be a true reproduction of the combination of the several tones and notes played simultaneously.

An'object of the invention is to devise improved 25 apparatus of a simple character so that one or more manual keyboards and pedal keyboards may be employed and so that various tones may be played in several registers or pitches.

A further object of the invention is to devise g0 suitable apparatus to obtain improved quality of tone when notes are played together, as'i'or example in chords.

In order that the invention may be more clearly understood it will now be described by way of 35 example with reference to the accompanying drawings in which:

Figure 1 shows diagrammatically the lower range of the great manual and an electrostatic generator.

Figure 2 shows diagrammatically range of the great manual and the ciated with this manual.

Figure 3 shows a side elevation '(in section) of a playing key.

Figure 4 shows aside elevation (in section) of a draw stop.

Referring now to Figures 1 and 2 which show when taken together the great manual of a musical instrument having for example two man- 50 ual keyboards designatedthe swell'manual and the great'manual and one pedal keyboard, it will be clear from Figure 1 that the electrostatic generator comprises a stationary disc I o! insulating material to the face of which are'secured the upper stops asso- .55 a number of concentric electrode rings 2 of conkey" . ducting material, the inner and outer sufaces oi each ring 2 being of sine wave formation.

There is provided one ring 2 for each semitone of the musical scale and the total number of rings will be dependent upon the number 01' 5 octaves of the musical scale which it is desired to produce; in the present example five octaves with extra rings for partials of the higher notes. The numbers of undulations of successive rings 2 progress geometrically in the ratio of 1 to the 12v'2 which is the interval 01a semi-tone oi. the tempered musical sca1e.-

A second circular disc 3 provided with concentric electrode rings of exactly similar construction to that already described is adapted to rotate 16 about the axis of the said rings so that the surfaces of both sets of rings are in close proximity. All the rings on the rotating disc are connected together and through the medium oi. a slip ring and'brush are connected to the negative or earth 20 lead 4 which is connected to one pole of the source of current 5 (Figure 2). Each ring or the stationary disc I is connected to one terminal of a condenser 6 having a capacity of ab ut .01 to .1 microfarad, the opposite terminals of the condensers being all connected to the grid it of the first valve or a suitable thermionic amplifier. Each ring 2 is also connected through a charging resistance 1 01' about 5 megohms to one terminal of a condenser 8 the other terminal of which is connected to the negative or. earth lead 4. In parallel with the condenser 8 is connected a leak resistance 9. The purpose of this condenser and leak resistance will be hereinafter described.

To the junction of the charging resistance 9 I with the condenser 8 are connected the ends l0,

II, I! of-three coupling resistances l3, H and I5 respectively, the other end of the coupling resistance I being connected to a regulating resistance l1, and also to the end of one or more key resistances which will be more fully described hereafter. The other ends of the resistances l3 and, I! are similarly connected to regulating resistances associated with the swell manual and the pedal keyboard and to the ends of one or more key resistances associated with the said manual and keyboard. The other end of the regulating resistance I! is connected to the negative or return lead. 4.

In the embodiment shown there are associated .with theplaying keys of the great manual twenty-four bus bars [Ba-Hz. Each playing (Figure 3) is adapted when depressed to actuate its associated key switch rod 20, which rod causes movement of a plurality of contacts 2| 5 so as to cause them. to e contact with said bus bars. mch contact 2i is connected through a resistance 22 to the appropriate ring 2 of the.

generator to be charged. It will be clear, thereit will-be clear that a larger number of partials may lie-employed for each note if desired. Furthen-stops of seven registers or pitches are also 7; employed, namely 16 It., 8 it., 5% ft., 4 ft., 2% it.-,

a. 2 ft. and 1% ft. The partials used are the j fundamental, second, third, fourth, fifth, sixth a'ndeighth. They are taken from notes in the,

' tempered musical scale. Thus, assuming the ld it. fundamental be-CCC, the seven harmonics 1| would be CCC, CC, GG, C, E, G, c.- The same noteis used for difierent pmials'in several tones and therefore inthe present example the number 'oi...bus-bars is twenty-four. I

It will -iie'clear that when two or more playing at keysare depressed in each of which a particular frequency forms the fundamental or a harmonic then the. potential of the charged member should rise'in accordance with the number of keys depressedand in accordance-with the desired in- 8 tensity of" this particular frequency. The leak resistance a together with the key switch resist- 22 perform this desiredfunction. In order the Potential across'the leak resistance 9 r increase in proportion to the potential of 6 the bus bars connected when a plurality of playing. keys are depressed and so'that the greatest additive be produced it is essential that the value of, the leak reslstaance 9 be less than theindlvidual-value o! the key switch resistances 22.

that the values of theresistances 22 are all'equalthen if x'equals the value of a resistance ,22 dividedby the value of the leak .re-

sistance and'N equals the number oibus bars at. anyone time to a ring 2 the perthe true additiveeflect of the .bus bar; potentials on the member will be as follows:

assesses assesses eases-ssasaaiass saassas It will thereiore be seen from the-last four:

i lines of the above table that it is' possible to obtain a greatly increased additive eii'ect and that when x is (according to the invention) greater thanunity and when several keys are depressed simultaneously the potential of the charged mem- 10 her increases substantially with the'number of keys depressed and that an approximately proportional to the addition of the several potentia'ls to'which the member may be connected.

-"rhe manner in which the bus bars law-I83:

. are to the necessary predeiiermiilefl P (flrst partiaD'of the note 04 in 8 its apo'asea v or quality of tone desired. If the first bus bar ifla be used for the fundamental of the 16 ft. pitch then the switches 26 are erranged to make contact as.iollows 16 it. 8. 5% it. 4 it. 2% it. 2 it. 1% it.

0 Partial Number-oi bus bar 18a 18b 18c 18d 181 18g lai 18b 18d 18f 18g 18] 18! 18B 18c 18! 1811 18] 18m 180 18p 18d l8 18 181 180 l8q 18S 183 181 180 1&1 18f; 18w 18! Each bus bar Ida-88x is connected to the negative or earth lead ii through a resistance 21 (Figure 2) and each stop switch contact 26 is connected to one end of a stop resistance 28 the other end of anyone of which may be connected to any one of twenty-one potentiometer bus bars 29. These bus bars 29 are connected to tapping points on a potentiometer resistance'iid one end of which is connected to the positive pole of the source of current 5, which may be a battery, the

negative pole of which is connected to the neg ative or earth return 5.

In order to vary the intensity of the sound produced by the instrument the positive end of the potentiometer in may be connected to the positive pole of thesource of current 5 through a variable resistance til the moving member 32 of which may be conveniently constructed as a pedal to be operated by the foot of theperiormer, which pedal corresponds to the swell pedal on an organ.

It will be seen from an inspection of the apparatus as above described that the potentials of the twenty-tour bus bars i8ci8z will vary according to the combination of stops 23 drawn (and to the chosen bus-bars 29 to which they have'been connected to preselect the individual tones of the pitch stopsas hereinafter described),

and when the playing keys it are depressed the rings 2 will be charged to suitable potentials for the desiredtone. On rotation of the disc 3 elecand converted into sound waves by the loud speakenj It will also be clear that the charging potential of each ring will depend upon the number of stops drawn and the number of keys depressedin each keyboard.

The condenser 8 associated with each ring maintains the charge on the ring alter the key or keys have been released and thereby produces a reverberation or-sustaining eilect, the length of time or which is governed by the values of the condenser 8 and resistance 9.

The energy or the sound produced willbe proportional to the square of the chargin P tentials Therefore to produce double the sound energy the charging potential must increase 1.414

4) are provided which,

slstances 28 times, which is equivalent to a gain 0! 3 decibels.

In previous instruments of this nature (see for example original U. S. Patent No. 1,996,669, Reissue 19,702, September 10, 1935), the charging potential to'anyone ring 2 is the resultof applying anumber of potentials to resistances connected in parallel and one member may short circuit several bus bars. The resulting potential is therefore only the mean of these potentials.

In the present invention whenever several potentials are applied to a'single electrode they are applied through resistances in parallel and the electrode is always connected through a resistance to the negative or earth lead and as previously'indicated the leak resistance in the case of an electrode must be less than each of the parallel key resistances.

I have also found that by correctly proportioning the resistances 21 and 28 it is possible to obtain a potential on the conductor or bus-bar which shall increase in the desired proportion in order to maintain the energy of the harmonic tones at their correct levels either when sounded singly or in combination. The parallel reshould' all be or the same value and as will be shown the value of the coupling resistance 21 between the conductor and the negative lead 4 should be less than that of a single parallel resistance 28.

If N be the number of parallel resistances, RI the value of each parallel resistance, R the value 01' the coupling resistance, then it can be shown mathematically that the gain in decibels is equal to:

Therefore in tential gain the value 01' R! I it should be from 1.5 to 3.0 according to the number of parallel resistances connected simultaneously.

In order easily to adjust the power and tone quality of the various stops the potential oi the potentiometer bus equal decibel increments. Thus if the increments are 3 decibels then the power of any stop can be increased or reduced by one half by simply connecting each stop resistance 28 to the next higher or lower potential bus bar 29. An individual harmonic in any stop can also easily be altered by a given amount and records can equally easily be kept of the levels of the various harmonics inany tone so that it can be repeated in manufacture.

The resistance of the with each manual (or be or low value in order stops are drawn and keys depressed the increased current shall not appreciably.- aflect the potentials oi the potentiometer bus bars..29.

potentiometer associated pedal keyboard) should It will be understood that the above described arrangements incorporated in the great manual may be employed for the swell manual and for the pedal keyboard. In the latter case the compass will, as is usual, be much smaller.

Although the invention nas been described in its application to a musical instrument in which the electrical vibrations are. generated electrostatically and the tempered inuslcal scale is employed both forthe fundamental and harmonic tones, I may, however, use the tempered scale order to obtain the correct po-.

bars 29 should increase inthat when a number of.

for the iundamental tones'and true harmonics instead 01 tempered harmonics.

Further, the invention is described as employing an electrostatic generator having rotating rings to generate the desired wave forms, but it 5 is to be understood that any other form of generator which is capable of generating undulating potentials of controlled frequency and wave form electrostatically may be employed.

I claim:

1. An electrical musical instrument comprising an electrostatic generator for producing electrical oscillations or undulating potentials of any desired frequencies and wave forms, comprising a plurality of condensers, one electrode of each of said condensers being adapted to be charged, and means for continuously varying the capacity/of each of said condensers in-accordance with said wave forms, a set of key contacts for controlling the charging of the charged electrodes, a source of potential, a set of controlling resistances and a set of key switch resistances, each electrode to be charged being connected to one pole of the source of potential by one of said controlling resistances, and being connected to each key contact appropriate to such electrode and to the other pole of the source of potential through a key switch resistance which is greater than the controlling resistance employed.

2. An electrical musical instrument comprising an electrostatic generator for producing electrical oscillations or undulating potentials of any desired frequencies and wave forms, comprising a plurality of condensers, one electrode of each of said condensers being adapted to be charged, and means for continuously varying the capacity of each of said condensers in accordance with said wave forms, a set of key contacts for controlling the charging of the charged electrodesja source of potential, a set of controlling resistances and a set of key switch resistances, one end of each key switch resistance being connected to the appropriate key contact, while the other ends of said resistances are connected together and to the electrode to be charged.

3. An electrical musical instrument comprising an electrostatic generator for producing electrical oscillations or undulating potentials of any desired frequencies and wave forms, comprising fa plurality of condenser one electrode of each of said condensers being adapted to be charged, and means for continuously varying the capacity of each of said condensers in accordance with said wave forms, a set of key contacts for controlling the charging of t e charged electrodes, a source of potential, a set of controlling resistances and a set of key switch resistances, each electrode to be charged being connected to one pole of the source of potential by one of said controlling resistances, and being connected to each key contact appropriate to such electrode-and tothe other pole oi the source of potential through a key switch resistance which is greater than the controlling resistance employed, a charging resistance being inserted in the conductor connecting the controlling resistance to the generator electrode to be charged.

4. Anelectrical musical instrument comprising anelectrostatic generator for producing electrical oscillations or undulating potentials of any desired frequencies. and wave forms, comprising a plurality oi condensers, one electrode of each of said condensers being adapted to be charged, and means for continuously varying the capacity of each of said condensers in accordance with said 75 wave forms, means icr supplying chargin P0- tentials to a set, of bus-bars associated with the playing keys of the instrument, a plurality of stops each of which actuates\ a' plurality of switches to energise said bus-barsto predetermined potentials from a source of current and means associated with said switches for controlling, proportioning and combining the sound energy of the various harmonic tones when a pluralityof keys are played simultaneously and a plurality of stops are drawn.

5. An electrical musical instrument comprising an electrostatic generator for producirigelectrical oscillations or undulating potentials of any desired frequencies and wave forms, comprising a plurality" of condensers, one electrode of each of said condensers being adapted to be charged, and. means for continuously varying the capacity of each ofsaid condensers in. accordance with said wave forms, means for, supplying charging bus-bars associated with the playing keys of the instrument, and switching means for energising selected bus-bars to predetermined potentials corresponding to ,,the requiredpitch or combination of pitches whereby musical sounds of the desired quality in the required pitch or combination of pitches are obtained when a plurality of keys are depressed simultaneously. r

6. An electrical musical instrument comprising an electrostatic generator for producing electrical oscillations or undulating potentials of any desired frequencies and wave forms, comprising a plurality of condensers,- one electrode of each of said condensersbeing adapted to be gharged, and means for continuously varying the capacity of each of said condensers in accordance with said wave' forms, means for supplying charging potentials to a set of bus-bars associated with the playing keys of the instrument, a plurality of stops each of whlch'actua-tes a plurality of switches to energlse said bus-bars to predetermined potentials from a source of current, said bus-bars being connected to one pole of the source of potential through resistances while any potentials supplied to said bits-bars are applied through parallel resistances-which are all of the same value and each of which is greaterthan each of said first-mentioned resistances.

v'7. An electrical musical instrument compris ing an electrostatic generator for producing electrical oscillations or undulating potentials of any desired freqllencies and .wave plurality of condensers, one electrode of each of said condensers being adapted to be charged, and means .for continuously varying the capacity of each of said condensers in accordance with said wave forms, means for supplying charging potenpotentials to a set'of 'tials 'to a' set of bus-bars associated with the playing keys of the instrument, a plurality of I sired frequencies forms, comprising a stops each of which actuates a plurality of switch* es to energise said bus-bars to predetermined potentials from a source of current and a potentiometer having tapping points being shunted across the source of current, the tappings being connected to said switches through resistances while further resistances are connected between each of the bus-barsand one pole of the source of current and having a variable resistance controlled by a pedal connected in series with said potentiometer' for varying the voltage across said potentiometer. v 8.- An electrical musical instrument comprising an electrostatic generator for producing electrical oscillations or undulating potentials of any desired frequencies and wave forms, comprising a plurality of condensers, one electrode of each of said condensers being adapted to be charged, and means for continuously varying the capacity oi each of said condensers in accordance with said wave forms, means for supplying charging potentials to a set of bus-bars associated with the playing keys of the instrument, a plurality of stops each of which actuates a plurality of switches to energise said bus-bars to predetermined potentials from a'source of current and a potentiometer being shunted across said source of current, the tappings being connected to stop switches through resistances, while further resistances are com nec ed between each of the bus-bars and one pole of t resistance being from 1.5 to 3 times the value or each resistance connecting a bus-bar to one pole of the source of current. v

9. ,An electrical musical instrument comprising an electrostatic generator for producing electrical oscillations or undulating potentials of any deand wave forms, comprising'a plurality of condensers, one electrode of anchor fsaid condensers being adapted to be charged, and

means for continuously varying the capacity of each of said condensers in accordance with said wave forms, a source of current, a plurality of bus-bars, a plurality of stops, a plurality of stop switches actuated by each of said stops to eon-' 4 tact selected ones of said bus-bars, means for energising said stop switches to predetermined potentials, a. plurality of playingkeys, a plurality of switches actuated by each key to. contact said bus-bars, resistances connected switches, a conductor connecting the resistances associated with selected ones of said playing key switches together, a coupling resistance connected to said conductor, acharging resistance connected to said coupling resistance and to the electrode to be charged, a regulating resistance connected to the junction of said coupling and charging resistances and to one pole of the source of current, 1 v

' ALBERT MIDGLEY,

at e source of current, the value of each stop to each of said 5 

